You can download X-15: Extending the Frontiers of Flight" by Dennis R. Jenkins free from NASA at this NASA site. They have even made it available in both Kindle and Sony eBook formats. I downloaded it from the web and then manually installed it on my Kindle. Kudos to NASA for truly advancing science by making it available to everyone.
North American built the X-15, a rocket-powered aircraft. It was the successor to the Bell X-1, which rocketed Chuck Yeager past the 'sound barrier' on 14 October, 1947. The X-15 was carried aloft by a B-52 to begin a flight. Once dropped, the pilot would ignite the rocket engines and shoot upward. Although the program was cancelled in 1968, it established manned aircraft speed records that have not been broken.
The X-15 is an interesting subplot in Tom Wolfe's The Right Stuff. If you recall, many of the nation's best pilots were excluded from NASA's first call from Astronauts. The criteria of a college diploma excluded such notables as Chuck Yeager. So while the big research dollars were diverted to would-be Buck Rogers such as Alan Shepard, Gus Grissom, and John Glenn, the Air Force continued to channel a few dollars to keep the X-15 program flying. (According to Wiki, John Glenn didn't have a diploma, but was still admitted.)
I just downloaded the eBook a few minutes ago, and have not yet read it. I did scan through the book (that is a little painful, as you know, on eBooks). One thing that caught my attention was a conclusion that the X-15 program would never survive today's bureaucratic, political, and risk-averse world. It just couldn't happen. Mmm. Our pioneer DNA must be getting stretched thin.